In the digital copying system, the dot area ratio per pixel is changed by means of a digital copying machine to form an electrostatic latent image according to the dot area ratio, and toner particles are then attached to the electrostatic latent image to form a toner image. In this case, the amount of the toner particles to be attached to the electrostatic latent image depends on the dot area ratio. Thus, the amount of the toner attached varies with the area ratio, enabling gradation recording to be effected in response to the amount of the toner attached.
However, most conventional copying machines have employed an analog system. Accordingly, the development of toners has been intended for analog copying machines. Thus, the development of black toners for digital copying machine has been slow.
In the case where conventional black toners for analog copying machine are used for digital copying machine, when the amount of the black toner developed is changed in response to the dot area ratio, a phenomenon occurs in which the optical density of toner image doesn't vary any longer, i.e., the optical density of toner image reproduced doesn't show linear change with respect to the change in the amount of toner developed, even if the amount of toner attached exceeds a predetermined value.
The reason for this phenomenon is believed to be because that since black toners for analog copying machine which have heretofore been used comprise carbon black in toner particles in a large amount as more than 6% by weight to enhance the optical density for the purpose of optimizing the gentleness, softness and letter reproducibility of toner image, the proportion of the background to be covered by carbon black is soon saturated in the region to which a small amount of toner is attached, resulting in the saturation of the image density of the toner image thus obtained.
Then, when it is attempted to solve the foregoing problem by reducing the content of carbon black in the toner particles, another problem arises. That is, as carbon black there has heretofore been used one having a relatively small particle diameter produced by furnace process. However, the content of carbon black in a toner comprising such carbon black with a relatively small particle diameter is reduced, the region in the toner particles free of carbon black increases, making difference between the transmittance of long wavelength light and short wavelength light. The resulting developed black image exhibits a brownish red tone, making it impossible to provide an excellent black reproducibility.